Which tank you choose depends on what battery you want to put it on as well. If you're using a small battery (ego/riva/etc), I wouldn't get a large tank. It will not look right and can be cumbersome. The 3.5ml DCT tank will look much better on a small battery. Both size
tanks are around $9-10 or you can get a large smoktech DCT replacement tank tube with caps for around $5 at a couple places. I've never seen the small replacement tanks as cheap as the larger ones.
808 and 510 have totally different thread patterns. While the carto BODY is the same size, you will need a thread adapter if you want to run one on the other. I would suggest just buying your tank at the same place you buy the adapter because it will save on shipping.
As for the best liquids....VG based juices tend to be thicker and won't wick quite as well, but there are many many people using high VG content juices in tanks. Not all VG juices are the same thickness due to how vendors dilute their VG. I use PG80 or PG70, but I suggest trying the percentage/
juice that you like and seeing how it goes.